[dm-devel] Re: [PATCH 3/3] Add timeout feature

[Andrew Morton]

On Fri, 27 Jun 2008 20:33:58 +0900
"Takashi Sato" <t-sato at yk.jp.nec.com> wrote:

> >>  case XFS_FSOP_GOING_FLAGS_DEFAULT: {
> >> - struct super_block *sb = freeze_bdev(mp->m_super->s_bdev);
> >> + struct super_block *sb = freeze_bdev(mp->m_super->s_bdev, 0);
> >
> > Using NULL here is clearer and will, I expect, avoid a sparse warning.
> 
> I checked it but I couldn't find a sparse warning in xfs_fsops.c.
> Can you tell me how to use NULL?

	struct super_block *sb = freeze_bdev(mp->m_super->s_bdev, NULL);

:)

It's much better to use NULL here rather than literal zero because the
reader of this code can then say "ah-hah, we're passing in a pointer". 
Whereas plain old "0" could be a pointer or a scalar.

We should always use NULL to represent a null pointer in the kernel. 
The one acceptable exception is when testing for nullness:

	if (ptr1)
		if (!ptr2)

Often people will use

	if (ptr1 != NULL)
		if (ptr2 == NULL)

in this case as well.  (I prefer the shorter version personally, but
either is OK).